Interface exchange coupling in Co nanoparticles dispersed in a Mn matrix

The structural and magnetic properties of 1.8 nm Co particles dispersed in a Mn matrix by co-depositing pre-formed mass-selected Co clusters with an atomic vapour of Mn onto a common substrate have been studied by using EXAFS (extended x-ray absorption fine structure), XMCD (x-ray magnetic circular dichroism), magnetometry, and theoretical modelling. At low Co volume fraction (5%) Co-Mn shows a significant degree of alloying and the well-defined particles originally deposited become centres of high Co concentration CoMn alloy that evolves from pure Co at the nanoparticle centre to the pure Mn matrix within a few nm. Each inhomogeneity is a core-shell particle with a Co-rich ferromagnetic core in contact with a Co-depleted antiferromagnetic shell. The XMCD reveals that the Co moment localized on the Co atoms within the Co-rich cores is much smaller than the ferromagnetic moment of the Co nanoparticles deposited at the same volume fraction in Ag. Electronic structure calculations indicate that the small magnitude of the core Co moment can be understood only if significant alloying occurs. Monte Carlo modelling replicates the exchange bias (EB) behaviour observed at low temperature from magnetometry measurements. We ascribe EB to the interaction between the ferromagnetic Co-rich cores and the antiferromagnetic Mn-rich shells.